A down-conversion mixer circuit translates the carrier frequency (fc) of an input RF signal to a lower carrier frequency. In a direct down conversion mixer, the output occurs at baseband, with a carrier frequency equal to zero.
Mixers typically display nonlinear properties. Even order nonlinearities translate strong interfering signals directly to baseband, which is particularly problematic in direct down conversion mixers. The even order distortion masks the wanted signal and thereby lowers the overall signal to noise ratio.
Communications receivers must process very strong unwanted signals without corrupting the oftentimes very weak desired signal. In a typical radio receiver, the down conversion mixer is the first block to operate at baseband frequencies and therefore has no filtering before it to minimize the strength of unwanted signals.
It would be therefore advantageous to have a direct down conversion mixer with low even order distortion.